IB/mlx5: Use ARRAY_SIZE instead of sizeof/sizeof[0]

Acked-by: Eli Cohen <eli@mellanox.com>
Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: Doug Ledford <dledford@redhat.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>